1. Field of Invention
This invention relates generally to gas-fired infrared heaters for processing food, and more particularly to an infrared dehydrator unit for minced fish to produce an edible food product.
2. Status of Prior Art
My prior U.S. Pat. No. 4,507,083 and my above-identified copending patent application disclose gas-fired heaters for projecting an infrared (IR) beam in a radiation pattern having a predetermined pattern or geometry for irradiating a food product to effect uniform heating thereof at a rapid rate.
The transfer of heat takes place by three processes: conduction, convection and radiation. In conduction, heat is transferred through a body by the short range interaction of molecules and/or electrons. Convection involves the transfer of heat by the combined mechanisms of fluid mixing and conduction. In radiation, electromagnetic energy is emitted toward a body and the energy incident thereto is absorbed by the body to raise its temperature. Radiant heating, therefore, differs from both convection and conduction heating, for the presence of matter is not required for the transmission of radiant energy.
According to the Stefan-Boltzmann law, the rate of heat transfer between a source of radiated heat whose temperature is T.sub.s and an absorbing body whose temperature is T.sub.b is equal to T.sub.s.sup.4 -T.sub.b.sup.4 ; that is, to the difference between the fourth powers of these temperature values. In convection heating, the rate of heat transfer is proportional only to the temperature difference between the body being heated and the surrounding atmosphere. Hence convection heating is inherently very slow, as compared to the nearly instantaneous effects of radiant heating.
Radiation heaters in present commercial use are of the infrared type, the infrared band of thermal radiation lying within the electromagnetic wave spectrum. The quality and intensity of radiation in the infrared band of 0.7 microns to 400 microns depends on the temperature of the radiating body. If, therefore, the radiating body is a refractory ceramic heated by a gas-fired jet burner, one can only accurately adjust the quality and intensity of the IR radiation if it is possible to carefully control the operation of the gas-fired burner.
Despite the fact that IR heaters are much more economical to operate and act with extreme rapidity, and IR heaters are therefore far superior in this regard to convection ovens for cooking or baking food, they have enjoyed limited success. The reason for this is that commercially available gas-fired IR heaters are relatively difficult to control and also give rise to an uneven heating action.
Effective infrared heating depends not only on the radiant source temperature but also on what is referred to as the "geometric view factor." This factor determines the relationship between the pattern of IR radiation and the surface of the product being heated. With the typical IR heating arrangement, portions of the product to be heated are more completely exposed to IR rays and will be heated more rapidly to a high temperature than those portions that are not as fully exposed. As a consequence, the product may not be properly heated.
To overcome these limitations, the infrared heater disclosed in my prior U.S. Pat. No. 4,507,083 is adapted to project an infrared beam in a radiation pattern having a predetermined geometry for irradiating the surface of the food product or other body to effect uniform heating thereof at a rapid rate. The heater includes a ribbon-type burner having an elongated pre-mix casing into which is fed air and gas, and an outlet extending along a slot in the casing and projecting therefrom. The outlet is provided with two sets of corrugated ribbons separated by a gas pressure chamber, whereby the air-gas mixture from the casing passes through one set into the chamber where the pressure thereof is equalized before the mixture passes through the other set from which it emerges as a sheet of flange of uniform intensity. The outlet is inserted in the longitudinal socket of a refractory body to impinge on a surface thereof whereby the surface is heated to a temperature level causing the surface to emit infrared energy which is projected by an array of radiation horns formed in the assembly.
The gas-fired infrared heater disclosed in my copending application is also adapted to project an infrared beam in a directional radiation pattern. The heater is constituted by a ribbon-type burner and a refractory body capable of radiating infrared energy when heated to an elevated temperature by the burner. The burner is formed by a metal pipe having a longitudinally-extending outlet defined by a pair of parallel plates projecting laterally from the pipe and having a set of corrugated ribbons therein, a pad of thermal insulation being secured to the outer surface of each plate, whereby when a mixture of air and gas is fed into the pipe and ignited, a sheet of flame emerges from the outlet.
The gas fueled burner is received within a longitudinally-extending internal channel in the refractory body, the outlet being then aligned with an internal cavity so that the flame emerging from the outlet impinges on a surface of the cavity to heat this surface to an elevated temperature causing it to emit infrared radiation. The cavity communicates with an array of openings in the body which form radiation horns to produce the desired directional infrared radiation pattern. Because the burner is thermally shielded in the channel, metal fatigue and deleterious distortion of the ribbons is avoided even at very high operating temperatures.
The present invention is concerned with the dehydration of raw fish in minced form to produce an edible food product. U.S. Pat. No. 4,405,653 to Gray (1983) discloses a dehydrated fibrous fish product adapted for human consumption and a process for producing this product which is prepared from raw fish or raw fish scraps, fresh or frozen. The fish is processed by a onestep, reduced pressure dehyration in the absence of air. The final form of the dehydrated product has a fibrous texture and is off-white in color. The product is unique in that the odor-producing bodies, fats and oils are not altered or extracted. When the product is rehydrated or reconstitute in water, it has the same nutritional value, odor and taste as does the whole fish.
As pointed out in the Gray patent, his product needs no refrigeration. It increases in weight by up to 80% when mixed with water and can be used to form the basis of a food by itself or it may be added to rice, flour, bread, potato, or other vegetables and foods as desired by market preference. It can be shaped or combined with other textures to be reconstituted into fillet form. It is especially useful in countries where refrigeration is in short supply.
The Gray process for manufacturing this food product begins with conversion of the raw fish material to a relatively uniform particulate size which is easily dried. The eviscerated whole fish and/or fish parts from commercial fresh fish filleting operations are subject to processing in a meat-bone separator whereby the fish flesh is removed from the bones and gristle. The fish flesh is in a minced form following this step. This minced fish and/or fish scrap from commercial frozen fish block cutting operations are then subjected to treatment in a vacuum (reduced pressure) dehydrating system. According to Gray, this use of reduced pressure is necessary since the final product characteristics cannot be met if oxygen (an oxidizing atmosphere) is present in a drying system.
As a practical matter, a fish dehydration technique that requires that a load of minced fish be vacuum dehydrated by vacuum drum drying, by freeze drying or similar means has serious drawbacks which militate against mass production of the food product; for this dehydration technique is costly and time-consuming as well as inefficient.
In vacuum dehydration, the load must be transported into a vacuum chamber and the air must be evacuated before heating is commenced. In the course of heating within the evacuated chamber, the evaporated moisture must be withdrawn from the chamber, and upon completion of dehydration, the load must be transported out of the chamber.
In heating a minced fish load to a temperature of up to 90.degree. C. as called for in the Gray patent, the walls of the tank or drum containing this load are heated to an elevated temperature and the pieces of fish tend to stick to these walls and become scorched. As a consequence, a significant percentage of the dehydrated fish is burned. The scorched pieces of fish lack the desired off-white color and therefore must be discarded. And since the walls of the tank have pieces of scorched fish sticking thereon, the tank walls must be scrubbed and cleaned before another fish load can be introduced therein.